In recent years, a lithium ion secondary battery used as a power source for a portable electronic device is a mainstream of non-aqueous secondary batteries. A lithium ion secondary battery generally comprises a positive electrode formed of a positive electrode current collector carrying a positive electrode material mixture, and a negative electrode formed of a negative electrode current collector carrying a negative electrode material mixture. The positive electrode material mixture includes a positive electrode active material formed of a composite oxide of a transition metal and lithium, a conductive material, and the like. The negative electrode material mixture includes a negative electrode active material formed of a carbon material and the like which can store and release lithium. Although a lithium ion secondary battery has high voltage and high capacity, an improvement is desired due to a decrease in capacity through charge and discharge (hereinafter referred to as “charge/discharge”) cycles.
As one of the causes of the decrease in capacity through charge/discharge cycles, a decrease in electron conductivity of an electrode can be mentioned. The decrease in electron conductivity is caused by a partial fracture of a conductive network structure formed of a conductive material due to expansion and contraction of a positive electrode through charge/discharge cycles. In order to secure sufficient electron conductivity even with the partial fracture of the conductive network structure, a content by percentage of the conductive material included in the electrode material mixture may be increased, or a content by percentage of a non-conductor included in the electrode material mixture, i.e., binder and/or thickener, may be decreased.
However, the conductive material itself does not contribute to a battery capacity, and a usage of the conductive material in great amount causes a reduction in the battery capacity. Thus, a decrease in the content by percentage of a binder and/or a thickener included in a positive electrode material mixture is desired.
Additionally, in view of decreasing an environmental burden, it is desirable to use water as a dispersion medium for an electrode material mixture slurry. The electrode material mixture slurry is prepared by mixing an electrode material mixture with a liquid component as a dispersion medium. The slurry is applied on a current collector to make the material mixture carried on the current collector. Thus, attempts to decrease the content by percentage of a binder and/or a thickener have been made, especially for the cases where water is used as a dispersion medium.
Conventionally, a fluorocarbon resin such as polytetrafluoroethylene (PTFE) is used as a binder (see Japanese Laid-Open Patent Publication No. Hei 08-339828). However, a fluorocarbon resin is low in viscosity, and its binding property is developed by chain entanglement. Therefore, when the content by percentage of a binder is decreased, the electrode material mixture tends to easily separate from a current collector, thereby causing a reduction in capacity.
Hence, there has been proposed a usage of a copolymer of 2-ethylhexyl acrylate, acrylic acid, and acrylonitrile as a binder (see Japanese Laid-Open Patent Publication No. Hei 11-25956). According to this proposal, a total amount of a binder and a thickener can be decreased to 2.6 parts by weight per 100 parts by weight of an active material. Also, an excellent cycle life is obtained even when a conductive material is decreased to 5 parts by weight per 100 parts of an active material.
However, when an amount of the materials other than an active material, i.e., a conductive material, a binder, and a thickener, to be mixed is decreased, gradually, it becomes difficult to uniformly disperse these materials in a liquid component which is to be a dispersion medium. Thus, an electrode material mixture slurry becomes unstable, and a conductive material becomes vulnerable to re-coagulation due to a shearing force while an electrode material mixture slurry is applied on a current collector and then dried. As a result, the weight of the applied electrode material mixture becomes non-uniform depending upon a position on the current collector.
Causes may vary, but mainly the unstable electrode material mixture slurry is caused by a great difference in surface free energy between a conductive material and a dispersion medium when using water as a dispersion medium. Between the materials having a great difference in surface free energy, affinity is low. When affinity between a conductive material and a dispersion medium is low, a conductive material becomes vulnerable to re-coagulation.
General preparing steps of conventional electrode material mixture slurry comprise one step: adding a liquid component to be a dispersion medium into a mixture including an active material, binder, and the like, all the amount together at once and then kneading the mixture. However, in the above step, because a sufficient shearing force is not given to the conductive material, dispersion state of the electrode material mixture in the obtained slurry and viscosity of the slurry easily become unstable. In order to prepare a stable electrode material mixture slurry, a method to disperse an electrode material mixture in a liquid component should be considered well.
In view of the above, the following have been proposed. Japanese Laid-Open Patent Publication No. Hei 9-213309 suggests that after kneading a conductive material, a binder, and a dispersion medium, an active material is added and then further kneaded. Japanese Laid-Open Patent Publication No. Hei 10-144302 suggests that after kneading a conductive material and a dispersion medium, an active material and a binder are added and then further kneaded. Japanese Laid-Open Patent Publication No. Hei 11-144714 suggests that after kneading a binder and a dispersion medium, an active material and a conductive material are added and then further kneaded. Japanese Laid-Open Patent Publication No. Hei 11-213989 suggests that after kneading a conductive material and a dispersion medium, an active material is added and further kneaded, and then a binder is added and kneaded.
However, in any of the above methods, since almost a total amount of a dispersion medium is admixed at once, an electrode material mixture slurry is kneaded without a sufficient shearing force being given to a conductive material. In such methods, it is difficult to stabilize a dispersion state of the electrode material mixture, viscosity of the slurry, and the like.
Additionally, the following have been proposed. Japanese Laid-Open Patent Publication No. Hei 7-161350 suggests that a dispersion medium is added, by adjusting its amount, to a mixture of a conductive material, a binder, and an active material in order to decrease an amount of necessary dispersion medium and to shorten the time for drying. Japanese Laid-Open Patent Publication No. 2000-353516 suggests that the same method as the above Publication No. Hei 7-161350 is conducted in order to improve a dispersion state of an electrode material mixture in the slurry, and a binding property of an electrode material mixture and a current collector. Japanese Laid-Open Patent Publication No. 2001-167756 suggests that a dispersion medium is dividedly added several times in trace amounts to a mixture of an active material, a conductive material, and a binder dissolved in a dispersion medium.
In any of the above proposals, an electrode material mixture slurry is kneaded in a state in which a shearing force is given to the conductive material, since a dispersion medium is dividedly added. However, in any of these proposals, an organic solvent which has high affinity with a conductive material is used as a dispersion medium. In the case where an organic dispersion medium is used, when an excessive shearing force is given to a mixture, a coagulation of a conductive material is adversely prompted. Therefore, in addition to a problem of organic solvent waste, there is a problem in that the amounts of a binder and a conductive material to be added to an active material can not be decreased.
Thus, there has been proposed a method in which water is used as a dispersion medium, and carboxymethyl cellulose (CMC), which is a water-soluble polymer, is used as a thickener, and an aqueous solution of the thickener is dividedly added several times to a mixture kneaded to become a thick paste (see Japanese Laid-Open Patent Publication No. 2000-348713). According to this method, in a step of applying the electrode material mixture slurry on a current collector, a yield rate can be improved since coagulation in the electrode material mixture slurry can be reduced.
However, it is difficult to reduce even invisible, fine coagulation in this method. In particular, there is a problem in that a weight of an electrode material mixture to be applied becomes non-uniform, depending on a position on a current collector, due to re-coagulation of a conductive material caused by a low affinity between the conductive material and a dispersion medium. As a result, capacity varies depending on a position on an electrode plate, and a cycle life of a battery will deteriorate. Such a problem becomes particularly noticeable when manufacturing an electrode plate by using an electrode material mixture slurry left for a while after preparation.
Also, conventionally, a thickener is dissolved in a liquid component to be a dispersion medium in advance, and used as a solution. However, in order to prepare a solution efficiently by prompting a dissolution of the thickener, which is usually hardly soluble, a powerful mixer such as a homogenizer is needed. Although a strong convection can be given to a mixture of a thickener and a dispersion medium by using a homogenizer and the like, there is a problem in that the molecular entanglement which is the base of the thickening effect of the thickener is destroyed and the thickening effect is decreased.